FUMED SILANIZED SILICA

Fumed, by means of dimethyldichlorosilane silanized silica characterized by the following physicochemical data: Grindometer value less than 20 µm Tamped density 25 to 85 g/l And obtained in that a fumed silica having the following physicochemical data: Bet surface area m2/g: 25 - 400 Average primary particle size nm: 5 - 50 pH: 3 -10 Carbon content %: 0.1 -10 Is silanized by means of dimethyldichlorosilane and then ground.

Full Text

Fumed silanized silica
The invention relates to fumed silanized silica, to a
process for preparing it and to its use.
Fumed silica (pyrogenically prepared silicon dioxide)
is known from Ullmanns Enzyklopadie der technischen
Chemie, Volume 21, page 464 (1982).
It is prepared by burning a vaporizable silicon
compound, such as silicon tetrachloride, for example,
in a mixture with hydrogen and oxygen.
The comminution of materials to form coarse powders
(50-500 µm) , fine powders (5-50 µm) and even greater
finenesses (less than 5 µm) is common and widespread
practice. For all comminution tasks there is a
multiplicity of technical and industrial equipment
offered and operated, all adapted to the particular
circumstances of the specific tasks. A good overview of
the comminution problems and of the diverse machines is
given in Ullmanns Enzyklopadie der technischen Chemie,
3rd Edition, Volume 1, pages 616 to 638.
For fumed silica the average primary particle diameters
are notably lower (5-50 nm) than can be obtained by
mechanical comminution.
The primary particles and aggregates of the fumed
silica with a surface area of 200 m2/g can be
visualized in the electron microscope.
The primary particles and aggregates of a fumed silica
agglomerate to form larger assemblies whose size is
generally in inverse proportion to the primary particle
size or in proportion to the specific surface area. The
agglomerate size also increases in line with the extent
to which the fumed silica is compacted.

The binding forces holding these agglomerates together
are relatively weak. Nevertheless, when these agglome-
rates are incorporated into and broken down in a liquid
system for the purpose of homogeneous distribution of
the primary particles and aggregates, or particles with
a low degree of agglomeration, a certain shearing
energy is required. Depending on the particular field
of application, dispersing is carried out using any of
a very wide variety of mixing devices, with determining
factors for the selection being not only the viscosity
and polarity of the system but also the agglomerate
strength and the desired homogeneity.
With simple agitator mechanisms, such as paddle
stirrers, it is usually not possible to carry out to
satisfaction the direct incorporation of small amounts
of silicas, especially not when the systems in question
are of low viscosity. However, paint and varnish
makers, and also those who carry out processing, have
an interest in obtaining performance-optimum distri-
bution of the silicas, which are used predominantly as
thickeners and thixotropic agents, by means of very
simple devices, with a very low energy input and in a
very short time.
In the case of paddle-stirrer dispersing, the coarse
silica agglomerates are not sufficiently comminuted and
hence are able to make only a small contribution to
raising viscosities and thixotropy. The data relate to
a UP resin (unsaturated polyester resin) as dispersion
medium.
Reducing the agglomerate size by dispersing outside a
liquid system, in other words, in practice, in the air,
or by grinding in the conventional sense, is possible
only to a limited extent, since in the case of material
with a given propensity to agglomerate the comminution
is followed immediately by the re-establishment of the

old agglomeration state. This effect comes about no
later than after recompaction of the material which as
a result of the mechanical intervention has undergone a
high degree of loosening and which in this form is not
amenable to dispatch and storage. The storage time as
well would have the effect of renewed agglomerate
enlargement.
A value taken as a dimensional number and evaluation
variable for the stage of distribution of a dispersible
silica and maximum agglomerate size of the dispersion
(granularity) is the so-called grindometer value to DIN
53203.
A known procedure is to render fumed silica hydro-
phobic, to grind it in a pinned-disc mill and then to
classify it (US 2004/0110077 A1) .
This known silica is used as an external additive in
toner mixtures.
Hydrophilic fumed silica with a BET surface area
possesses a grindometer value as determined in UP resin
(unsaturated polyester resin Ludepal P6 from BASF, 2%
dispersion) in accordance with the DIN specification,
of 50 to 60.
If this fumed silica is also relatively highly
compacted (100 to 120 g/l), the grindometer value as
well is also significantly higher, specifically more
than 100, whereby necessitating an additional, not
inconsiderable energy as a thickener and thixotropic
agent.
A known procedure is to grind a highly dispersed silica
having a surface area of approximately 300 m2/g in a
pinned-disc mill.

The grindometer value achieved is initially, for the
uncompacted silica, 25.
If this silica is compacted to 50 g/l, the grindometer
value rises to 30, and in the case of further
compaction to 75 g/l it rises to as far as about 40.
In the case of storage over a period of three months,
the ground silica, not modified but compacted to
50 g/l, has a grindometer value of 50 to 60.
Reagglomeration can only be prevented in accordance
with the prior art if the hydrophilic silica is mixed
with 3% by weight of a hydrophobic silica and if this
mixture is ground by means of an air-jet mill or a
pinned-disc mill (EP 0 076 377 Bl).
In this case, for a fumed silica having a BET surface
area of 200 m2/g, even after compaction to 73 or
107 g/l, a grindometer value of 35 is achieved.
For a fumed silica having a BET surface area of
300 m2/g, the addition of hydrophobic silica prior to
grinding produces a grindometer value of 10 for a
tamped density of 28.1 g/l and of 15 to 20 for a tamped
density of 50 g/l.
The known fumed silicas have the disadvantage that they
contain an unwanted fraction of hydrophobic silica.
The invention provides a fumed silanized silica which
is characterized in that it has the following physico-
chemical data:
Grindometer value less than 20 |im
Tamped density 25 to 85 g/l
The invention further provides a process for preparing
the silica of the invention which is characterized in

that silanized, structurally modified fumed silicas
which are characterized by groups fixed on the surface,
the groups being dimethylsilyl and/or monomethylsilyl,
preferably dimethylsilyl, are ground.
In one preferred embodiment of the invention the
silicas used can have the following physicochemical
data:
BET surface area m2/g: 25 - 400
Average primary particle size nm: 5 - 50
pH: 3-10
Carbon content %: 0.1-10
Fumed silicas are known from Winnacker-Kuchler
Chemische Technologie, Volume 3 (1983) 4th edition,
page 77 and
Ullmanns Enzyklopadie der technischen Chemie, 4th
edition (1982), Volume 21, page 462.
Fumed silicas are prepared in particular by flame
hydrolysis of vaporizable silicon compounds, such as
SiCl4, for example, or organic silicon compounds, such
as trichloromethylsilane.
The silanized, fumed silicas used in accordance with
the invention can be prepared by treating fumed silica
in a known way with dimethylchlorosilane and/or mono-
methyltrichlorosilane, the groups dimethylsilyl and/or
monomethylsilyl being fixed on the surface of the fumed
silica.
In one particular embodiment of the invention the
initial silica used can be a fumed silicon dioxide
which has been hydrophobicized by means of
dimethyldichlorosilane.
The grinding of the silanized fumed, silica can take

place by means of a pinned-disc mill or an air-jet
mill.
The silica of the invention shows no propensity to
reagglomerate. The grindometer value is below 20.
The fumed silica of the invention can be used as a
filler in silicone rubber compounds.
Silicone rubber compounds and the use of fumed silica
(AEROSIL®) in silicone rubber compounds are known
(Ullmann's Encyclopaedia of Industrial Chemistry,
Volume A 23, Rubber, 1, 221 ff.; Rubber 3, 3,6 ff.;
Volume A 24, Silicones 57 ff. 1993).
Fumed silica is used on account of its excellent
thickening effect (thixotroping) in silicone sealants,
this thickening effect being desirable in the context
of use as a jointing compound.
Where, however, the silicone rubber compounds are to be
used as coating materials, a low level of thickening is
desired (US 6,268,300) .
Of decisive importance in both cases is the optical
quality of the surface of the silicone vulcanizates.
It is an object of the present invention, therefore, to
provide silicone rubber compounds which through the use
of fumed silica as filler exhibit an optically high-
grade surface after vulcanization.
The invention provides silicone rubber compounds
containing 0.5% to 60% by weight, based on the total
mass, of the fumed silica of the invention having the
following characteristic physicochemical data:
Grindometer value less than 20

Tamped density 25 to 85 g/l
and
40% to 99.5% by weight, based on the total mass, of an
organopolysiloxane of the formula

where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl
radicals, having 1 to 50 carbon atoms, unsubstituted or
substituted by O, S, F, Cl, Br, I, in each case
identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 40-10 000 repeater units.
Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime,
alkoxyamido, alkenyloxy, acryloxy or phosphate
radicals, it being possible for the organic radicals to
carry up to 20 carbon atoms, and in each case identical
or different.
Z' = oxime, alkoxy, acetoxy, amino, amido,
n = 1-3
x = 100-15 000.
As organopolysiloxane it is possible to use all poly-
siloxanes which have or have been able to be used to
date as a basis for room-temperature-crosslinking
(room-temperature-vulcanizing) (RTV) compositions. They
may be described for example by the general formula

where x, R, Z' and Z have the following definitions:
where R = alkenyl, alkoxy, aryl, oxime, acetoxy, alkyl
radicals, having 1 to 50 carbon atoms, unsubstituted or

substituted by 0, S, F, Cl, Br, I, in each case
identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 40-10 000 repeater units.
Z = OH, Cl, Br, acetoxy, amino, amineoxy, oxime,
alkoxyamido, alkenyloxy, acryloxy or phosphate
radicals, it being possible for the organic radicals to
carry up to 20 carbon atoms, and in each case identical
or different.
Z' = oxime, alkoxy, acetoxy, amino, amido,
n = 1-3
x = 100-15 000.
Within and/or along the siloxane chain in the formula
indicated above it is also possible for there to be
other siloxane units present, usually only as
impurities, in the form of diorganosiloxane units, for
example those of the formula RSiO3/2, R3O1/2 and SiO4/2, R
in each case having the definition indicated for it
above. The amount of these other siloxane units ought
not to exceed 10 mol per cent.
Examples of R with the definition alkyl radical are,
for example, methyl, ethyl, propyl, hexyl and octyl
radicals; possible alkenyl radicals are vinyl, allyl,
ethylallyl and butadienyl radicals; and as aryl
radicals it is possible to use phenyl and tolyl
radical.
Examples of substituted hydrocarbon radicals R are in
particular halogenated hydrocarbon radicals such as
3,3,3-trifluoropropyl radical, chlorophenyl and
bromotolyl radical; and cyanoalkyl radicals, such as
the P-cyanoethyl radical.
Examples of polymers as radical R are polystyrene,

polyvinyl acetate, polyacrylate, polymethacrylate and
polyacrylonitrile radicals which are attached to
silicon via carbon.
On account of the greater ease of access the
predominant fraction of the radicals R is composed of
methyl groups. The other radicals R are, in particular,
vinyl and/or phenyl groups.
Particularly in the case of the presence of formula-
tions which are storable in the absence of water and
which cure to elastomers at room temperature on ingress
of water, Z and Z' are hydrolysable groups. Examples of
such groups are acetoxy, amino, amineoxy, alkenyloxy
(for example H2C = (CH3CO-)), acyloxy and phosphate
groups. Primarily on account of the greater ease of
access, preferred groups Z are acyloxy groups,
especially acetoxy groups. Excellent results, however,
are also achieved using, for example, oxime groups,
such as those of the formula -ON=C (CH3) (C2H5) , as Z.
Examples of hydrolysable atoms Z are halogen and
hydrogen atoms; examples of alkenyl groups Z are, in
particular, vinyl groups.
The viscosity of the organopolysiloxanes used in the
context of the invention ought not to exceed 500 000 cP
at 25°C, preferably 150 000 cP at 25°C. Accordingly the
value x ought preferably not to exceed 40 000.
Examples of organopolysiloxanes which can be used are,
for example, the silicone polymers E50 (α, ω-hydroxydi-
methylsiloxypolydimethylsiloxane) or M50 (α, ω-hydroxy-
dimethylsiloxypolydimethylsiloxane) from GE Bayer
Silicones.
Mixtures of different organopolysiloxanes can also be
used.
The mixing of these organopolysiloxanes with the fumed

silica and, where appropriate, with the further
constituents of the formulation of the invention can
take place in any desired known way, for example in
mechanical mixing devices. It is accomplished very
rapidly and easily, irrespective of the sequence in
which the mixing constituents are added.
Preferably the fumed silicas of the invention are used
in amounts of 0.5% to 60% by weight, preferably 3% to
30% by weight, based on the total weight of the
compounds which can be cured to elastomers.
If the only reactive terminal units present in the
diorganopolysiloxanes which contain reactive terminal
units are those having Si-bonded hydroxyl groups, then
these diorganopolysiloxanes must be crosslinked. This
can be done in a conventional way by means of the water
present in the air, with the addition where appropriate
of further water, with a crosslinking agent. Here it is
possible for example to use the Silopren crosslinker
3034 from GE Bayer Silicones, the ethyltriacetoxysilane
optionally in the presence of a condensation catalyst
in a known way. Suitable catalysts for all formulations
of the invention are, for example, the Silopren
catalysts DBTA or type 162 dibutyltin diacetate or
dilaurate from the same manufacturer.
In one particular variant of the silicone rubber
compounds of the invention it is possible additionally
for there to be 0.5%-20%, preferably 2%-10% by weight,
based on the total weight of the compound, of a
crosslinker having the formula
R'4-tSi Z'4
with R = alkyl, alkoxy, acetoxy, oxime, aryl, alkene
radicals, having 1 to 50 carbon atoms, unsubstituted or
substituted by 0, S, F, Cl, Br, I, in each case

identical or different, and/or polystyrene, polyvinyl
acetate, polyacrylate, polymethacrylate and polyacrylo-
nitrile radicals having 5-5000 repeater units,
Z' = OH, Cl, Br, acetoxy, oxime, amino, amineoxy,
alkenyloxy or phosphate radicals, it being possible for
the organic radicals to carry up to 20 carbon atoms, in
each case identical or different, and
t = 3 or 4.
All weight data relate to the total amount of silicone
rubber compounds.
Examples of silanes of the formula indicated above are
ethyltriacetoxysilane, methyltriacetoxysilane, iso-
propyltriacetoxysilane, isopropoxytriacetoxysilane,
vinyltriacetoxysilane, methyltrisdiethylaminooxysilane,
methyltris (cyclohexylamino)silane, methyltris(diethyl-
phosphato)silane and methyltris(methylethylketoximo)-
silane.
Of course it is possible for formulations of the inven-
tion to contain, besides organopolysiloxanes, hydro-
phobicized silica, crosslinking agents and crosslinking
catalysts, if desired, fillers which are conventionally
used mostly or frequently in compounds which can be
cured to elastomers. Examples of such substances are
fillers having a surface area below 50 m2/g, such as
coarse quartz powder, kaolin, phyllosilicates, clay
minerals, diatomaceous earth, additionally zirconium
silicate and calcium carbonate, and also untreated
pyrogenically produced silicon dioxide, organic resins,
such as polyvinyl chloride powders, organopolysiloxane
resins, fibrous fillers, such as asbestos, glass fibres
and organic pigments, soluble dyes, fragrances,
corrosion inhibitors, curing retardants, such as benzo-
triazole, and plasticizers, such as dimethylpoly-

siloxanes end-blocked by trimethylsiloxy groups.
Optionally the RTV 1K [one-component] silicone rubber
compounds of the invention can contain 0.1%-20%,
preferably 0.1%-15%, with particular preference 0.1%-
10% by weight (based on the total amount of the
formulation (of water-binding substances) . Suitable
substances for this purpose are, for example,
carboxylic anhydrides, for example acetic anhydride or maleic anhydride, and/or carbonic esters, such as for
example diethyl carbonate, ethyl carbonate and/or
alkenyloxy compounds and/or ketals, such as dimethyldi-
oxolane, for example. It is possible to use one or more
of these substances.
Additionally the silicone rubber compounds may contain
0.01% to 99.5% by weight of an unfunctionalized
polysiloxane. Here it is possible to use the poly-
siloxanes already specified, provided that they are not
functionalized. One suitable, non-functional poly-
siloxane is, for example, Baysilone oil M1000 (polydi-
methylsiloxane) from GE Bayer Silicones.
Additionally the silicone rubber compounds may contain
0.01% to 6% by weight of organic or inorganic compounds
of the metals Pt, Sn, Ti and/or Zn as catalyst and/or
0.01% to 6% by weight of inhibitors and/or 0.01%-6% by
weight of fungicides and/or bactericides and/or 0.01%
to 6% by weight of adhesion promoters (such as, for
example, Silopren adhesion promoter 3001 from GE Bayer
Silicones, with the composition: di-tert-butoxydi-
acetoxysilane). As fungicides/bactericides it is
possible for example to use isothiazolinone, Vinycin or
benzisothiazolinone.
The silicone rubber compounds of the invention can be
used as silicone rubber systems from the group of the
room-temperature-vulcanizing one-component (1K RTV)

silicone rubber sealants and also self-levelling roorn-
temperature-crosslinking silicone rubber compounds
(1K RTV).
The silicone rubber compounds can be used as jointing
compounds, window sealants, seals in motor vehicles,
sports equipment and household appliances, heat-
resistant seals, oil-exuding and chemical-resistant
seals, and water-vapour-resistant seals, and seals in
electrical and electronic appliances.
The silicone rubber compounds can be used as coating
materials for textiles, e.g. lace tape (antislip) , and
textile materials, e.g. woven glass fabric or woven
nylon fabric.
The vulcanizates of the silicone rubber compounds of
the invention advantageously have a high-grade surface.
The inventive examples were produced by metering
commercial AEROSIL® R 972 (bagged product) into the
mill employed, using a metering balance, and subjecting
it to grinding. The physicochemical data of the
AEROSIL®972 are listed in Table 1.
The parameters of the production process are listed in
Table 2.
The experiments were carried out using a pinned-disc
mill (Alpine 160Z, rotor diameter 160 mm) or an air-jet
mill (grinding chamber diameter: 240 mm, grinding
chamber height: 35 mm). The ground product was isolated
with a hose filter (filter area: 3.6 m2, filter
material: woven nylon fabric). In further experiments
the ground product obtained was packaged into
commercially customary bags using a commercially
customary bagging machine. In further experiments the
bags packed with ground product were levelled prior to

palletization, using a method routine in the industry
and suitable for the purpose.

Table 3:
Physicochemical data of the inventive silicas and the
comparative example

With virtually the same specific surface areas and
unchanged pH values, the data of the ground products
exhibit lower grindometer values. Surprisingly the
lower grindometer values are retained in spite of the
compaction, evident through the tamped density, as a
result of bagging or bagging/levelling.
In some cases the tamped densities are in fact above
that of the oxide used, i.e. the oxides of the
invention, despite the same or even higher compaction,
exhibit lower grindometer values.

DA = Particle diameter, averaged over surface area
DV = Particle diameter, averaged over volume
D50 (g) = Median value, weight distribution
The inventive fumed silica can have a D50 (g) (i.e.
median value, weight distribution) of 25.7 to 35.0 ran.
The total span of the particles can be 2500 to
87.140 nm.
The particle diameter averaged over the surface area,
DA, can be 23.0 to 30.9 nm.
The particle diameter averaged over the volume, DV, can
be 26.5 to 40.0 nm.
Figures 1 to 8 show the graphical representation of the
distributions, measured on the silicas of Example 1 (in
accordance with the invention) and of the comparative
example.

The silica of the invention can have a d50 value as
determined by Cilas of 4.5 to 7.0 µm.
BET surface area
The BET surface area is determined in accordance with
DIN ISO 9277.
Tamped density
The tamped density is determined in accordance with DIN
EN ISO 787-11.
Principles of tamped density determination:
The tamped density (formerly tamped volume) is equal to
the ratio of the mass to the volume of the powder after
tamping in a tamping volumeter under defined condi-
tions. According to DIN ISO 787/XI the tamped density
is reported in g/cm3. Owing to the very low tamped
density of the oxides, however, we state the value in
g/l. Furthermore, the drying and sieving, and the
repetition of the tamping process, are omitted.

Apparatus for tamped density determination:
Tamping volumeter
Measuring cylinder
Laboratory balance (reading accuracy 0.01 g)
Tamped density determination procedure:
200 + 10 ml of oxide are introduced into the measuring
cylinder of the tamping volumeter so that there are no
cavities remaining and so that the surface is
horizontal.
The mass of the sample introduced is determined to an
accuracy of 0.01 g. The measuring cylinder containing
the sample is inserted into the cylinder holder of the
tamping volumeter and tamped 1250 times.
The volume of the tamped oxide is read off to an
accuracy of 1 m1.
Evaluation of tamped density determination:

Dispensette, 100 ml
Procedure for determining pH:
The determination takes place in a modification of DIN
EN ISO 787-9.
Calibration: Prior to pH measurement the meter is
calibrated using the buffer solutions. Where two or
more measurements are carried out one after another, a
single calibration is sufficient.
4 g of oxide are pasted in a 250 ml glass beaker with
48 g (61 ml) of methanol and the suspension is diluted
with 48 g (48 ml) of water and stirred for five
minutes, with a pH electrode immersed, using a magnetic
stirrer (speed about 1000 min-1) .
After the stirrer has been switched off the pH is read
off after a standing time of one minute. The result is
reported to one decimal place.
Grindometer value
Principles:
The degree of dispersion determines the performance
properties of the liquid thickened with Aerosil. The
measurement of the grindometer value serves to assess
the degree of dispersion. By the grindometer value is
meant the boundary layer thickness below which the
particles or aggregates present become visible on the
surface of the sample which has been coated out.
The sample is coated out in a groove with a scraper,
the depth of the groove at one end being twice the size
of the diameter of the largest Aerosil particles, and
decreasing steadily down to 0 at the other end. On a
scale indicating the depth of the groove, the depth
value is read off, in micrometers, the value in
question being that below which a relatively large

number of Aerosil particles becomes visible as a result
of bits or scratches on the surface of the binder
system. The value read off is the grindometer value of
the system present.
Apparatus and reagents:
Hegmann grindometer with a depth range of 100-0 micro-
meter.
Polyester resin dispersion with 2% Aerosil, prepared
according to Testing Instructions 0380.
Procedure:
The grindometer block is placed on a flat, slip-proof
surface and is wiped clean immediately prior to
testing. The Aerosil dispersion, which must be free
from air bubbles, is then applied to the deepest point
of the groove in such a way that it flows off somewhat
over the edge of the groove. The scraper is then held
by both hands and placed, perpendicularly to the
grindometer block and at right angles to its
longitudinal edges, with gentle pressure, onto the end
of the groove in which the dispersion is located. The
dispersion is then coated out in the groove by slow,
uniform drawing of the scraper over the block. The
grindometer value is read off no later than 3 seconds
after the dispersion has been coated out.
The surface of the spread dispersion (transverse to the
groove) is viewed obliquely from above at an angle of
20-30° (to the surface). The block is held to the light
in such a way that the surface structure of the spread
dispersion is readily apparent.
The grindometer value read off on the scale is the
value in micrometers below which a relatively large
number of Aerosil particles become visible as bits or
scratches on the surface. Individual bits or scratches
occurring randomly are not taken into account in this
context.

The granularity is assessed at least twice, in each
case on a newly spread dispersion.
Evaluation:
From the measured values the arithmetic mean is formed.
The relationship between the grindometer value in
micrometers and the FSPT units and Hegmann units, which
are based on the inch system, is as follows:
B = 8-0.079 A
C = 10-0.098 A = 1.25 B
In this relationship:
A = Grindometer value in micrometers
B = Grindometer value in Hegmann units
C = Grindometer value in FSPT units
II. Preparation of silicone rubber compounds
1. General experimental procedure
a) Principles
In order to test the performance properties of
AEROS1L® in RTV1 silicone sealants,
corresponding silicone compounds are prepared
on the laboratory scale to a standard
formulation.
b) Apparatus
The planetary dissolver must meet the following
requirements:
The stirring vessel has a capacity of
approximately 2 litres and is provided with a
jacket with cooling-water connection. Planetary
drive and dissolver drive are independent.
There must be a vacuum pump present. An
additional drum press makes product transfer
easier. Disassembly for cleaning purposes

should be rapid.
c) Formulation
62.4% silicone polymer
Silopren E 50 (GE Bayer Silicones)
24.6% silicone oil
Silicone oil M 1000 (GE Bayer Silicones)
4.0% acetate crosslinker
Crosslinker AC 3034 (GE Bayer Silicones)
1.0% adhesion promoter
Adhesion promoter AC 3001 (GE Bayer Silicones)
0.01% dibutyltin diacetate catalyst
8.0% fumed silica
AEROSIL® (Degussa AG)
d) Procedure
468.0 g of silicone polymer, 184.5 g of
silicone oil, 30.0 g of crosslinker, 7.5 g of
adhesion promoter
are weighed out into the stirring vessel and
homogenized for 1 minute with a planetary drive
speed of 50 rev min-1 and a dissolver speed of
500 rev min-1.
Thereafter 60 g of silica are incorporated at
the same speed in 2 lots (each about 30 g) and
the time required for wetting is measured.
As soon as the silica is fully wetted, a reduced
pressure of approximately 200 mbar is applied and
dispersion is carried out for 5 minutes with the
planetary stirrer at 100 rev min-1 and the dissolver
drive at 2000 rev min-1.
A drum press is used to transfer the sealant into two
aluminium tubes.
The silicone rubber compound obtained in this way is

coated out using a doctor blade and vulcanized at room
temperature in ambient air within 24 h. The surface of
the vulcanizates is assessed visually and rated in
accordance with a school grade system:
Grades: 1 = very good, 2 = good, 3 = satisfactory, 4 =
unsatisfactory, 5 = deficient
When the silicas from Examples 1, 3, 4, 6, 7 and 9 are
used, surprisingly, good surface properties of the
silicone vulcanizates are obtained, in comparison to
standard material, despite the fact that these silicas
in some cases have very high tamped densities, which
would normally lead to a poor surface quality. The
surface of the silicone vulcanizate with the standard
material is no more than satisfactory.